This invention relates to a scanner for deflecting a light beam for scanning by a rotating polygonal mirror.
Recently laser printers using a laser have been developed. One such laser printer, as shown in FIG. 1, comprises a semiconductor laser 1, a rotating mirror scanner 2, a drum 4 having a photoconductor surface 3, etc. In this laser printer, a laser beam generated from the semiconductor laser 1 is converged by a beam compressor 5 and directed toward a modulator 6. In the modulator 6, the laser beam is modulated in intensity in accordance with an electrical signal applied to the modulator 6. After passing through the modulator 6, the laser beam is projected on a polygonal mirror 8 of the scanner 2 through a beam expander 7. The laser beam reflected by the reflecting surface of the polygonal mirror 8 is projected on the photoconductor surface 3 of the drum 4 through a projection lens system or f.theta. lens system 9. As the polygonal mirror 2 is rotated at a constant speed, the laser beam reflected by the reflecting surface of the polygonal mirror 8 is deflected, and the photoconductor surface 3 is scanned by the laser beam. Since the laser beam is modulated in intensity by the modulator 6, and since the drum 4 is rotated in a fixed direction at a constant speed, a latent image, such as a pattern or characters, is formed on the photoconductor surface 3.
The rotating mirror scanner 2 has an advantage over a deflector using an electro-optical crystal in that it deflects a laser beam at a greater deflection angle. In order to obtain high resolution on a laser printer, it is generally necessary that the laser beam be deflected at a high deflection speed by the rotating mirror scanner 2. In other words, the rotational frequency of the rotating mirror scanner 2 to determine the deflection speed must be high, preferably 10.sup.4 rpm, for example. The rotating mirror scanner 2 is expected to rotate not only at a high frequency, but also with high accuracy and stability. Beside these requirements, the rotating mirror scanner must meet the following requirements:
(1) The polygonal mirror should not be contaminated by spindle oil or the like while rotating.
(2) Rotating parts should not be subject to complicated vibration modes or, in particular, high-frequency vibration during rotation.
(3) Less friction torque loss even at the start and during steady-state rotation.
(4) Long life.
(5) High portability.
(6) Ease of balance correction.
In a conventional motor unit which attempts to satisfy these requirements, the spindle is rotatably supported by bearings of a hydrodynamic pressure type. The dynamic pressure type bearings may be classified into two subtypes: herringbone dynamic pressure type bearings and tilting-pad dynamic pressure type bearings. The bearings of both these types cannot, however, be readily manufactured, and are unfit for mass production. As a thrust bearing for the spindle, a permanent magnet bearing of an uncontrolled or repulsion type is disclosed in Japanese Patent Publication No. 53-6854. This magnetic thrust bearing is an independent unit, and cannot readily attain static balance by itself. If subjected to an external dynamic force, the thrust bearing varies in its rigidity according to the direction of the vector of the working force which is unsettled. Thus, the thrust bearing is low in stability and rigidity. A controlled magnetic thrust bearing combining a permanent magnet and a coil has advantages over an uncontrolled one in both stability and rigidity. Since it requires peripheral equipment, however, the controlled magnetic thrust bearing complicates the construction of the motor, and is not suitable for low cost manufacture.
A more essential problem is that the rotational accuracy of the scanner might be decreased by a process for correcting the rotational balance of the rotating part of the scanner. In the prior art rotating mirror scanner, the rotor of the motor is mounted on a spindle between a pair of hydrodynamic bearings. A polygonal mirror is attached to one free end of the spindle extending from one bearing, while a magnetic thrust bearing is provided at the other free end of the spindle extending from the other bearing. In general, when the rotor as the rotating part to be rotated with the spindle, the polygonal mirror, and the magnetic thrust bearing are mounted on the spindle, their balance is corrected by a balancing machine. In the scanner with this structure, the spindle mounted with the balanced rotating part cannot be set in the hydrodynamic pressure type bearings as it is. In setting the spindle in the bearings, therefore, the rotating part is once removed from the spindle, then only the spindle is fitted in at least one of the bearings, and finally the rotating part is mounted on the spindle. Thus, the rotational balance of the scanner may possibly be lost which would lower the rotational accuracy after the assembly. In the prior art printer system, moreover, the scanner and the f.theta. lens or other optical system are provided independently, so that it is difficult to align the optical axis of the f.theta. lens with that of the scanner.